1. Field of The Invention
This invention relates to a pilot operated directional control valve system for use in an automatic transmission mounted in a construction vehicle or the like such as, for example, a dump truck, a motor scraper, etc., and more particularly to a device for detecting a malfunction or wrong operation of a solenoid operated valve operatively associated with the pilot operated directional control valve.
2. Description of The Prior Art
The pilot operated directional control valves have so far been employed in speed change controls of automatic transmissions of construction vehicles or the like. Each of the control valves of the kind specified is adapted to control the pilot pressurized fluid flow pass through in a valve spool thereof by means of a solenoid operated valve operatively associated with the control valve to allow the valve spool to slide in the control valve, so that a flow direction of pressurized fluid for engaging or disengaging a clutch mounted in the transmission is controlled. Accordingly, if a malfunction or wrong operation of the solenoid operated valve occurs, it becomes impossible to slide the valve spool of the control valve. Consequently, correct change-over operation of pilot operated directional control valves becomes unable to be obtained.
It is difficult, however, to check if the solenoid operated valve functions correctly in the valve control system so far employed. Particularly, in a multi-speed-stage automatic transmission provided with a multiplicity of solenoid operated valves, it is extremely difficult to detect which of the solenoid operated valves is malfunctioning. Moreover, in the multi-speed-stage automatic transmission, one or two solenoid operated valves are employed for each speed-stage, and some of the solenoid operated valves are commonly used for two or more speed-stages. Therefore, it is difficult to find out which of the solenoid valves is malfunctioning when a correct speed change operation cannot be made, and eventually all the solenoid valves must be disassembled, in turn, for checking.
Furthermore, it has been known in the art that 95% or more of the malfunctions of solenoid valves are due to the intrusion of foreign matters contained in the pilot fluid between the valve body and the seat thus making it impossible to achieve sealing or cut off function of the pilot fluid flow therethrough.